A capacitor microphone normally includes a stretched diaphragm, backplate, and spacer separating the diaphragm from the backplate. Typically, the diaphragm and backplate are constantly charged and form a charged capacitor. When an incident sound pressure excites motion of the stretched diaphragm, the spacing between the diaphragm and backplate changes, resulting in a corresponding change in the microphone capacitance and thus the voltage between the diaphragm and the backplate. This voltage change constitutes the microphone output signal.
Two common techniques are used to maintain the diaphragm and backplate to be constantly charged. The first technique involves biasing the microphone with a high voltage, called a "polarization" voltage. In accordance with this technique, the conducting diaphragm and backplate are connected to the high voltage through a large resistor so that the charge thereon is maintained by the high voltage. Because of the required biasing, such a microphone circuit can be undesirably expensive or large in size.
The other technique utilizes an electret, a metallized insulating foil, for the diaphragm. In accordance with this technique, the electret is pre-charged to have static charge trapped therein. The electret design is desirably simple and inexpensive. However, electret microphones have a sensitivity ("gain") which is directly proportional to the quantity of the trapped charge, and this quantity is subject to thermally activated Boltzmann detrapping processes. As a result, the electret microphones can exhibit a slow and an irreversible decrease in sensitivity over time and/or with increasing temperature.
A microphone array includes a number of individual microphones (or sensors) whose outputs are processed to produce a combined output, and is often used for providing directionality (i.e., acute sensitivity in selected directions) by virtue of the geometry of the configuration of the individual microphones. Unfortunately, the microphone array usually exhibits performance problems relating to gain non-uniformity among the individual microphones. Such gain non-uniformity may be attributed to non-uniform spacing between the diaphragm and backplate of each capacitor microphone. However, the spacing non-uniformity is inherent in the manufacture of the microphones. In the case where the individual microphones are of the capacitor type described above, the non-uniform spacing problem may be pronounced as the spacing in question is required to be narrow to begin with. Any small deviation in the spacing from one microphone to another results in a substantial gain difference.
Another contributing factor to the gain non-uniformity is the variability of the tension of the individual microphone diaphragms, which is also inherent in the manufacture of the microphones. Thus, in order to have the microphone array perform effectively, each individual microphone in the array needs to be calibrated before its use to afford a uniform gain. The required calibration is painstaking, time-consuming, and expensive as additional electronics providing the adjustment is needed. The calibration may require a complex acoustical test procedure as well.
Moreover, the above calibration needs to be repeated each time when the gain non-uniformity problem resurfaces due to, for example, changes in the tension of the individual microphone diaphragms over time. In addition, if the diaphragms are electrets, the quantity of the trapped charge therein is subject to the Boltzmann detrapping processes as mentioned above, increasing the chance of recurrence of the problem.
Accordingly, there exists a need for an inexpensive capacitor microphone array suitable for directional applications, whose design is conducive to providing a uniform gain by each individual microphone therein.